A media tray in an image forming apparatus may be equipped with a stack height sensor to detect the presence, absence, or quantity of media contained therein. It is also useful to particularly detect discrete states within the range of stack heights. For instance, sensors may be used to indicate full, intermediate, and empty conditions so that informational and operational warnings may be provided. One intermediate state of interest is a low condition. Low warnings are useful to determine whether enough media remains in the media tray to complete a print job. The same low warning may also be used to alert users of the condition so that they can add media before the media tray becomes completely empty. An empty condition signal is useful to alert users and, in some cases, prevent operation of the image forming apparatus to prevent damage or unnecessary wear. Some stack height sensors use a single sensor for each discrete height. For instance, two separate sensors may be used to generate a signal indicative of the low and empty conditions. Unfortunately, for these types of systems, stack heights other than these discrete positions will be unknown and unavailable.
Other stack height indicators use a continuously variable sensor that provides a signal that changes in proportion to the amount of media remaining in the media tray. These continuously variable sensors can provide stack height values over the entire range of heights. However, since most media sheets used in an image forming apparatus are thin in relation to the height of a stack, it is difficult to precisely determine when the discrete conditions are encountered. The output of a continuously variable sensor generally does not change a large amount as the height or position of a media stack changes as individual sheets are removed or added to the stack. Thus, systems that use a continuously variable sensor look for an expected range of sensor outputs to simulate discrete states.
Space limitations make integrating these components into an image forming apparatus increasingly difficult. Consequently, design and manufacturing constraints sometimes permit only one or another type of stack height sensor.